<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width,initial-scale=1">
    <title>Bootstrap</title>
    <link rel="stylesheet" href="bootstrap.min.css" crossorigin="anonymous">
    <style>
.loginRow{
    width: 100%;
    color: #fff;
    text-align: center;
    margin: 0 auto;
    background-color: #40de1c;
}
@media screen and (min-width:601px){
    .loginRow{
        width: 500px;
        margin: 0 auto;
        border-radius: 20px;
    }
}
    </style>
</head>
<body>
<div class="loginRow">
    <h1>登录框</h1>
</div>
</body>
</html>